Touchpads have been well known and widely used in various electronic products. A touchpad could serve as a simple, light and low-cost pointing device, such as one in a notebook for mouse control. FIG. 5 shows a perspective diagram for illustrating the operational principles of a capacitive touchpad, which comprises an insulation plate 48, a ground plane 36, a layer of Y trace 38, and a layer of X trace 40, and the later three are all coupled to a control circuit 46. An equivalent capacitor 42 exists between a Y trace 38 and the ground plane 36, and an equivalent capacitor 44 exists between an X trace 40 and the ground plane 36. When a finger or a conductive object touches on the insulation plate 48, upon the instant capacitance variation resulted therefrom, the control circuit 46 could determine the location of the finger or conductive object and a further processing accordingly.
Currently, there are three types of touchpad, i.e., resistive touchpad, electromagnetic touchpad, and capacitive touchpad. The capacitive touchpad has been applied in Internet public telephone and guiding system, but not provided with the functions of graphical input and handwriting input. The resistive touchpad has been applied in personal digital assistant (PDA) and electronic dictionary for example, but incapable of acquiring an input with fingers lightly touching thereon. Current tablet PC and electronic schoolbag are equipped with touchpad having handwriting input and key input functions integrated together, but only the resistive and electromagnetic touchpads are available for those applications. However, the resistive touchpad requires a concentrated pressing point for input thereon and thus is readily wear out, and the electromagnetic touchpad requires a special and battery-powered input pen for operations therewith.
In view of performance and cost, the capacitive touchpad is evidently superior to the resistive and electromagnetic touchpads. The operational principles of the capacitive touchpad reside in that an instant capacitance effect resulted from the touch of a finger or a conductive object on the touchpad is used to determine the touching location. Unlike the electromagnetic touchpad, the capacitive touchpad does not need to be operated with the aid of a power-consuming input pen, and the capacitive touchpad has a longer lifetime than the resistive touchpad, since there is no need for a concentrated pressing point thereon for input. Additionally, the capacitive touchpad has simpler construction, less elements and higher yield rate, and therefore the cost for mass production is lower.
On the other hand, for current electronic products, a newly developing trend is towards graphical input, and the need therefor is popularizing and growing. For example, the short message service (SMS) on mobile phone system has transited from text-only to text and graphics combined. Therefore, it is desired a touchpad having smaller volume, lower cost, and easier manipulation, and integrated with a graphical input function.